The present invention relates to a speaker driver and, more particularly, to a speaker driver used in a paging receiver or a pager operated by a battery.
FIG. 1 shows a general arrangement of a pager operated by a battery. This pager comprises a receiving circuit 205 for receiving a radio paging signal, an ID ROM 207 for storing a self-identification number (ID), a display 208 for displaying a destination telephone number or a message, a speaker 202 for generating an alert tone, a speaker driver 201 for driving the speaker 202, a decoder 206 for detecting the self-identification number in response to outputs from the receiving circuit 205 and the ID ROM 207 and supplying a display instruction to the display 208 while supplying an alert tone generation instruction to the speaker driver 201, and a DC/DC converter 209 for receiving power from a battery 203 having an internal resistor 204 and supplying a DC current to each part such as the decoder 206 excepting the speaker driver 201 and the speaker 202.
The speaker driver 201 for receiving power from the battery 203 parallelly with the DC/DC converter 209 is constituted by cascade connecting three transistors Q.sub.1, Q.sub.2 and Q.sub.3. The speaker 202 serves as a load of an output transistor Q3.
When the input transistor Q.sub.1 is ON/OFF-operated in response to an alert tone generation instruction from the decoder 206, the speaker 202 generates a sound corresponding to a repeating frequency of the ON/OFF operation.
Note that the alert tone generation instruction from the decoder 206 has a rectangular waveform having a frequency of several kHz, and a plurality of such instructions are output at predetermined intervals.
In the speaker driver 201 in FIG. 1, a voltage V.sub.B at a point 210 during a speaker drive mode, i.e., when an alert tone is generated, is varied in accordance with consumption of a current such as a speaker driving current and a voltage drop V.sub.DROP across the internal resistor 204 of the battery 203.
A current consumed when an alert tone is generated is large and is not significantly decreased even if a power source voltage is dropped. However, since the resistance of the internal resistor 204 of the battery 203 is increased as the battery capacitance is decreased, the voltage drop V.sub.DROP tends to be increased.
FIG. 2 shows the voltage V.sub.B. The level of the voltage V.sub.B is dropped in every alert tone generation period indicated by S. Finally, a voltage obtained by subtracting the voltage drop V.sub.DROP from the voltage V.sub.B is decreased to a level below a minimum operation voltage V.sub.MIN, which is a minimum value of the power source voltage required for assuring the operation of the DC/DC converter 209 or the receiving circuit 205.
In other words, in a conventional pager using the speaker driver 201, the battery must be replaced with a new one when the voltage V.sub.B =V.sub.MIN +V.sub.DROP. Theoretically, a battery can be used until its voltage value is decreased to the minimum operation voltage V.sub.MIN. In practice, however, a battery replacement time becomes earlier by a time corresponding to the voltage drop V.sub.DROP.
The conventional pager will be explained with reference to an example. The minimum operation voltage V.sub.MIN in a pager of this type is set at 1.0 V. Generally, the voltage drop V.sub.DROP is about 0.2 V. A battery replacement voltage becomes 1.2 V.
FIG. 3 shows a discharge characteristic of a SUM-3P battery. The terminal voltage becomes 1.2 V in about 41 hours, and 1.0 V in about 56 hours. The service life of the battery is shorter by about 15 hours than it should be.
In a pager of this type using a battery, the service life is an important parameter determining a commercial value of the apparatus, thereby posing a serious problem.